76 AKACHNIDA. 



front of them (E). Mark regarded the spherical structures found behind the 

 nuclei in the median eye of Scorpio, the so-called " phaospheres " of Ray 

 Lankestei:, as remains of the original rods, while the rods now found in front 

 of the nuclei (E, st) represent new formations. Since, however, the "phao- 

 spheres" are found in the lateral eyes of Scorpio which do not arise through 

 inversion (Ray Lankester, Carriere) this view is not easy to maintain. 



If we compare the two theories of the origin of the Arachnid eyes, we ought 

 to take into consideration, as an important factor, that the compound (facet) 

 eyes are built up on a convex base, while the Arachnid eyes have a concave base, 

 and in this character more nearly resemble the simple eyes. This feature is 

 wanting in the compound eyes of Limulus, which have as a base an almost level 

 surface. 



The attempt just made to bring the various Arachnid eyes into relation with 

 one another was undertaken to reconcile the facts revealed by ontogeny with the 

 structure of the adult eye. Physiological factors ought, perhaps, to have been 

 taken more into account. It must be clearly understood that the above is 

 merely an attempt to facilitate the comprehension of the Arachnid eyes, till, 

 through further research, we have attained a more exact knowledge of their 

 structure, and the ontogenetic processes through which they pass, which are 

 still in many respects very obscure. The literature on this subject is so 

 extensive, that we have not attempted to acknowledge the views of different 

 authors in the way done in other parts of this work. It has thus been possible 

 to treat our subject more freely. 



The Respiratory Organs. 



The Lungs. The two lungs of the Dipneumones arise in the 

 form of two wide depressions at the bases of the limbs of the 

 second abdominal segment (Salensky, Bruce, Morin). Their further 

 development (Schimkewitsch, Locy) somewhat resembles that already 

 described for the lungs of the Scorpiones (p. 19). 



The lung-sacs are directed forwards in relation to the stigmata. 

 At the anterior end, especially in the ventral part of the sac, 

 infoldings arise ; these are the leaves (formation of the lung-book). 

 The space between the two halves of each leaf is in direct continuity 

 with the body-cavity, so that the blood freely enters the lung-leaves. 

 The two lamellae are connected by cellular transverse trabeculae, no 

 doubt arising from the mesoderm (Locy) ; these are also present in 

 the adult (Fig. 40), and are then said to be of a muscular character 

 (MacLeod). On the free surfaces of the leaves, i.e., on those turned 

 to the cavity of the pulmonary sac, a cuticle is secreted, which, on 

 the surfaces directed ventrally, is homogeneous and of equal thick- 

 ness, while on the dorsal surfaces it is stronger, and beset with small 

 tooth-like thickenings (Locy); this distinction between the two 

 surfaces holds in the adult also (Fig. 40). 



It has already been shown (Vol. ii., p. 358) that the lungs of the Arachnida 

 show great morphological resemblance to the gills of the Xiphosura. Kingsley 



